The present invention relates to a method of controlling the setting pressure of a mine roof support, whereby the setting pressure is equal to the adjusting pressure of an overpressure valve communicating with the interior of the prop of the mine roof support for protecting the latter against excessive stresses, as well as to an arrangement for carrying out the method.
In modern operations of advancing an underground mining gallery, the roof of the mining gallery is supported by a hydraulically operated advanceable mine roof support. Such a mine roof support is advanced at short steps toward the front of the mine gallery as the latter is driven forwardly. During such advance the hydraulically operated props of the mine roof support are partly collapsed and, after the finished advancing step, again expanded so that the props will engage the roof of the mine gallery. The engagement of the props with the roof of the mine gallery is usually designated as setting of the mine roof support.
During the mining of the mineral, for instance coal, the roof of the mine gallery sags. The mine roof support is not supposed and cannot prevent such sagging of the mine roof, but it should take up such movement to act with a constant high force onto the mine roof, so that the sagging movement of the latter does not proceed in an uncontrolled manner to thereby endanger the operators working in the mine gallery. The hydraulic of the mine roof support must therefore be constructed in such a manner to permit a shortening of the props during the unavoidable movement of the mine roof and to act against such shortening of the prop with a constant high force. The hydraulic props have therefore to discharge during partial collapse of the same the thereby displaced pressure fluid. The discharge of such pressure fluid is carried out over overpressure valves, which will assure that the hydraulic pressure in the props is maintained constant to thus provide a continuous pressure against the mine roof. In general one has to distinguish between the force at which the props are set, that is the setting force with which the props support the mine roof during action of the overpressure valve and the adjusting force. In mining operations one speaks therefore of the setting pressure and the adjusting pressure. The adjusting pressure of the overpressure valves is subjected to variations due to tolerances, whereby these variations of the adjusting pressure unavoidably increase during longer use of the overpressure valves.
Usually the hydraulic circuits of hydraulic controls of mine roof support are constructed in such a manner that the adjusting pressure of the overpressure valves in the hydraulic circuit is 10 to 50% greater than the aforementioned setting pressure. This will assure that the overpressure valve during setting of the prop is not flown through by the amount of fluid provided by the pump connected to the hydraulic circuit. If the overpressure valves would be continuously flown through by the fluid stream supplied by the pump, the overpressure valves would be damaged in a very short time.
For reason of better control of the mine roof it is, however, desired to eliminate the pressure difference between setting pressure and adjusting pressure. Such pressure differences between setting pressure and adjusting pressure result in a non-uniform support of the mine roof and cause additional cracks, steps and breaks in the mine roof.